Method of rinsing metallic surfaces with solutions containing hexavalent chromium



2,970,935 METHOD or RINSING METALLIC SURFACES WITH SOLUTIONS CONTAINING HEXAVALENT CHROMIUM This invention relates to improvements in the method of operating hexavalent chromium containing rinse solutions on a continuous basis and more particularly relates to improvements in the method of formulating dilute hexavalent chromium containing solutions and continuously operating such solutions as rinses for improving the corrosion resistance of metallic surfaces and chemically coated metallic surfaces.

The utility of the hexavalent chromium ion in aiding the corrosion resistance of metal surfaces is well known and particularly in the art of chemically coating metals with a phosphate, oxalate, sulphide or similar coating, the use of a final dilute aqueous rinse containing hexavalent chromium ion is widely practiced. US. Patent 2,067,216 taught the use of solutions of this type for the purpose of improving the receptivity of a chemically coated metal surface for paint and specifically included the use of mixtures of chromic acid and phosphoric acid. Since this teaching, a large number of specific hexavalent chromium ion containing solutions have been used, each varying slightly from the other and compounded to achieve optimum improvement in corrosion resistance or paint base receptivity, for specific types of metallic surfaces or chemically coated metallic surfaces. Phosphate coatings, for example zinc phosphate andalkali metal phosphate coatings are soluble in certain hexavalent chromium containing solutions and in fact can be completely stripped from a metallic surface by a highly acidic hexavalent chromium containing. solution. For this reason it has been customary to carefully control the acidity of the dilute aqueous hexavalent chromium con- 'taining rinse solution, and in. continuous operation the maintenance of such a solution has been'etfected by periodically determining the free acid, concentration of t the rinse solution... As the rinse is used, sufiicient water is added to maintain the rinse solutions bath level and chrornic acid is added to preserve thefr'ee'acid at approxi-. mately its original concentration. It has been found that in certain installations this procedure u'poncontinued. use resultslin the rinse causing staining of the, surface, and in other installations the identicalprocedufe ul'tirhately results inthe rinse losingitsfefiectiveness enhancing corrosion resistance or improving f'the recptivity of the surface for paint, lacquer or the like.

accordance w'ith this invention itmhas now been found that the above difiicul'ties caii be'. completely avoided, and the continuous operation of dilute aqueous hexavalent, chromium containing rinse solutions rendered uniform in effectiveness by concurrently controlling the. free acidity, the total acidity and the hexavalent. chronlium ion concentration of such solution, instead or merely controlling free acidity "thereof. The" basic discovery upon whichthe present invention r'es'ts" is that thefaddition of water to the operating rinsesolution has? an important effect on the free acidityof therinse solution depending upon the. 'alkalinityq of that water which is added; Heretofore it has been suggested that the use of hard water in rinse" solutions of this type should be avoided but in accordance withthis invention it has 7 been found'that a much. more importantncharacteristic.

of these replenishihg waters is the alkalinity of the Cab water. The expression "alkalinity of the water refers to the amount of acid solution which is required to neutralize the water to a pH close to that desired for use as a rinse solution. This alkalinity figure is conveniently expressed in parts per million of calcium carbonate and is determined for any particular Water by titrating a 100 ml. sample of the water with N/SO H 80 using methyl orange as the indicator, which procedure titrates the water of a' pH of approximately 4.3. The titration in ml. of H 80 multiplied by 10 is the alkalinity in parts per million of calcium carbonate. This alkalinity figure can be used to formulate a desired hexavalent chromium ion containing rinse to contain any given desired concentration of CrO and having the desired free acidity and total acidity. The makeup of such solutions will be discussed more in detail hereinbelow.

The. free acidity of the dilute aqueous hexavalent chromium ion containing rinse solutions of this invention should fall within the range of about 0.05 to about 1.0. As used in this specification and in the appended claims, the expression free acidity refers to the number of ml. of N/ 10 sodium hydroxide which is required to titrate a 25 ml. sample of the rinse solution to a bromcresol green end point. The number of ml. is the points.

of free acidity. The solutions of this invention preferably have a free acidity within the range of about 0.l-0.5

and the most consistent results are attained when the skilled in the art, results from the uneven concentration of the hexavalent chromium ion on a localized portion of a surface being rinsed and can be visibly detected as a slight yellow to yellow color. Such a stain is particularly undesirable for surfaces which are to be subsequently painted becausethestain shows through the subsequent paint layers as a different shade of the resulting final painted surface. While the resistance to corro sion, which is obtained, is not due solely to the concentrationof the hexavalent chromium ion in the solution,

it is necessary for consistent results to insure that the I concentration of the hexavalent chromium ion does not go below a minimum threshold value. It will also be apparent that as the minimum threshold value is approached themaintenance of the solution above the threshold, as hexavalent chromium ion is depleted from the n'nse solution both as a result of interaction with the metal surface anddra'gout, is a difficult matter and requires constant attention. It istherefore desirable to operate at a concentration substantially above the mum value. The rinsesolution should have a. hexavalent chromium ion concentration, expressed as percent of crO jweight/volume, of at least about 0.01%,and

the concentration preferably is maintained Within the range of about 0.04%-0.05%. The upper concentration of CrO is, as above indicated, the concentration which is slightly below-thatwhichwill cause staining for the particularwork being. processedtherethrough and for conventional commercial. applications a concentration as high as about 0.1% ispermissible. Where the treated surface is not to besubsequently coated with paint'or the like and staining is of lessconcern, the concentration ofqchromic acid may exceed the 0.1% conce'ntrati'on- When the free acidity approaches or exceedsthe upper. l1m1t of about 1.0, certain chemical coatings andparticular ly zinc phosphate coatings are susceptible to being stripped and in this eventthe rinse solution becomes 0 contaminated with the zinc phosphate stripped coating matenal s. In addition, to consistently, maintaining the" free acidity and 'chromic acid concentration within the of phosphoric acid and chromic acid.

above given ranges, it is important in order to obtain consistent corrosion resistance enhancement to operate the solution within a total acid range of about 05-100 and preferably in the total acid range of 3-7, As used, in this specification and in. the appended claims, the expression total acid refers to the number of ml. of N/ sodium hydroxide which is required to titrate a 25 ml. sample of the time solution to a phenolphthalein endpoint and the number of ml. so used is the total acid point value of the solution.

1 The hexavalent chromium ion concentration of the nose solutions of this invention is preferably obtained by HSll'lg chromic acid to formulate the solutions, but the hexavalent chromium ion can be obtained by employing any of the metallic dichromates, such as, for example, sodium, ammonium. calcium, zinc, aluminum, and cobalt dichromates, etc. ,The use of metallic dichromates or an admixture with CrO is preferred in the material used to replenish the rinse solutions in this invention, and is necessary in all cases in which the addition of only chromic acid to return the operating rinse solution to substantially. its original free acidity will cause the hexavalent chromium ion concentration to decrease in the solution. In such case, a portion of the hexavalent chromium ion, at least, is added as a metallic dichromate. The alkali metal dichromates being relatively inexpensive and easily obtainable, are preferred for this purpose, but it will be appreciated that the use of other metallic ions may be more desirable in certain instances, depending upon the type of chemical coating which may be present on the surface that is being rinsed. For example, the provision of the hexavalent chromium ion as zinc dichromate is desirable when the chemical coating on thesurface being rinsed is a zinc phosphate coating, etc.

The free acidity of the solution may be adjusted as needed, by adding to the solution either chromic acid or phosphoric acid and as above indicated when the hexavalent chromium ion concentration has decreased below that desired,- the replenishment of the hexavalent chromium ion concentration by using chromic acid is preferred. However, when the restoration of the free acidity with chromic acids will cause the hexavalent chromium ion to exceed the desired level, the free acidity may be restored by adding phosphoric acid or a mixture of phosphoric acid and chromic acid and for this purpose a desirable admixture contains approximately equal parts It is also to be understood that in instances in which the replenishment of hexavalent chromium ion concentration of the solution with .CrO raises the free acidity above the desired level, that the free acidity which is 4 0.6 gm./1 of CrO and this solution has a CrO content of 0.06%. 'It' will be apparent; therefore, that in the operation of a rinse solution in a location in which the water added to maintain the rinse solution level is substantially free of alkalinity, as' is distilled or deionized water, that maintenance of a CrO concentration in the preferred range of 0.2%0.4% CrO by replenishing with CrO will soon cause the free acid value to rise and finally will cause the free acid to exceed the upper limit of 1.0. and use of this material will strip chemical coat ings on the metal surfaces being rinsed. If this practice is continued the contaminating dissolved coating will additionally cause the total acid to exceed its upper limit of 10.0. Such rinse solutions are satisfactorily continuously maintained, however, and within the above given limits by adding CrO in the form of a metallic 'dichromate, or an admixture of CrO and a metallic dichromat'c .which contains insufiicient CrO to detrimentally raise the free acid content of the solution.

A rinse solution which is replenished with highly alkaline water is preferably maintained by the addition of phosphoric acid together with the replenishing hexavalent chromium ion containing material. The amount of ph-osphoric acid which is necessary is that quantity which is required to neutralize the alkalinity in the replenishing water to a pH of about 4.3, and for this purpose an ideal replenishing material is a mixture of phosphoric and chromic acids. 1

In the continuous operation of rinse solutions which are replenished with alkaline water the alkalinity of the water can be satisfactorily counteracted by employing quantities of 75% phosphoric acid, per liter of the alkaline water, in proportion to the alkalinity of that water with reference points as follows: for 100 ppm. alkalinity water add 0.25 gm. of 75% H PO for 200 p.p.m.. alkalinity water add 0.5 gm. of 75% H PO for 300 p.p.m. alkalinity water add 0.75 gm. of 75 H PO and for 400 p.p.m. alkalinity water add 1.0 gm. of 75% H PO The addition of these quantities of phosphoric acid per liter of the water used to restore the rinse solution level has been found to completely counteract 'the effect of alkalinity on the maintenance of the rinse solution in uniform corrosion resistance enhancement condidesired can be obtained :byadding a metallic carbonate In preparing'n dilute aqueoushexavalent chromium ion containing rinse; solution to a particular desired concentration of CrO and a specific free acidity and total acid-' ity, the first step is to establish the alkalinity of the water; to be employed. It is only necessary then to add suflicient phosphoric acid tothe particular quantity of water employed in the particular solution to neutralize the alkalinity of the water to a pH of about 4.3 or, as above indicated, to the methylorarige endpoint. The desired hexavalent chromium ion concentration, expressed as- CrO isthen obtained by adding suflicient dichromat'e or dichromate mixedwith Cr0 to the neutralized'solution to secure that desired concentration. The effect of the alkalinity of the water on the free acidity of the resultingsolution' is clearly illustratedin the following example.- The addition of 0.1 gm./1 of CrO to distilled water produces a solution containing 0.01% CrO and having a free acid of 0.3 point. In order to obtain a solution having the same free acid when the water is one having an alkalinity of 300 p.p.m. it is necessary to add tion' as it is used to rinse either bare metal surfaces or chemically coated metal surfaces. In either type of rins-' ing operation, some of the chromic acid is used up in reaction with the metal surface or withthe chemical coating on that surface and some of the CrO is lost by drag-out on the surface being coated. In practice it has been found that sufficientCrO is removed in this'manner.

that when the alkalinityof the water is counteracted completely by the phosphoric acid addition, as above explained, a highly satisfactory replenishing material for restoring the Cr O concentration toits desired level, is one containing 10-25% CrO and'9075% of a metallic d ichromate, preferably calcium dichromate. 'The addi-, tlon of a quantity of this hexavalent chromium ion containing admixture sutficient to restore the Q03 level to its original concentration, additionally restores the free acidity which was lost during rinsing byvirtue of both drag-out and reaction with thesurface, as aboveexplained. i v Y 1 Q Q The rinse solutions of this invention can be applied any of the conventional procedures including spraying, brushing or flowing the solution on the surface to,

be rinsed, or the article can be dipped in the solution at temperatures between room and slightly below boiling,

preferably atabout F.- F; The replenishing materials are added at intervals sufficiently short so that preferably near to their initial values. V

The following specific example illustrates, in'greater detaih thel'orig'inal makeup and the replenishing of a hexavalent chromium ion containing rinse solution in accordance with this invention, but it is to be understood that the specific concentrations of the ingredients and the conditions used therein are illustrative only and are not intended to be definitive of the operating limits of this invention.

The water to be used in the rinse solution was first analyzed and found to contain 150 parts/million of alkalinity. A 1,000 gallon solution was prepared by admixing 3.12 lbs. of 75% HP O and 4.57 lbs. of an admixture containing 75 calcium dichromate and 25% CrO by weight, with sufficient water to form 1,000 gallone. Upon analysis the solution was found to have a CrO content of 0.04%, a free acidity of 0.24 point and a total acidity of 2.0 points.

Steel sheets, in the form of automobile fenders, ultimately intended to be painted, were first coated with a zinc phosphate coating of conventional type and thereafter rinsed in the rinse solution by spraying at 120 F.- 130" F. with conventional spray apparatus. At the end of 4 hours operation, the rinse solution had been replenished with 100 gallons of 150 ppm. alkalinity water and an analysis indicated that the solution then had a free acidity of 0.13 point, a total acidity of 1.9 points and a CrO concentration of 0.035%. The rinse solution was replenished by adding thereto 0.31 pound of 75 HP O and 5.6 pounds of the calcium dichromate- CrO admixture used in the makeup of the time solution. An analysis of the rinse solution after replenishing, indicated that the free acidity had been restored to 0.24 point, the total acidity was 2.3 and the CrO concentration was restored to the original 0.04%. Continued operation of the rinse solution under similar conditions required subsequent replenishing with similar quantities of 75% HP O and the admixture of calcium dichromate and C for each 100 gallons of water added to maintain the level of the rinse solution. The addition of. these quantities to the rinse solution maintained its free acidity and its CrO concentration at substantially the initial values and the quality of the receptivity for paint remained high and was completely satisfactory.

What is claimed is:

1. A method for maintaining an aqueous acidic hexavalent chromium containing solution in condition to enhence the corrosion resistance of metallic parts being continuously processed therethrough which comprises the steps of establishing an aqueous solution consisting essentially of about 0.01% to about 0.1% CrO and having a free acid in the range of about 0.05 to about 1.0 and a total acid in the range of 0.5-10.0 and as Water is added thereto during use, maintaining the free acidity thereof within the above given range by adding thereto an acid selected from the group consisting of chromic acids and phosphoric acids, maintaining the CrO concentration within the above given range by adding thereto at least one hexavalent chromium ion-containing material and operating said rinse within the total acid concentration range of 0.5-10.

2. A method for maintaining an aqueous acidic hexavalent chromium containing solution in condition to enhance corrosion resistance of chemically coated metallic parts being continuously processed therethrough which comprises the steps of establishing an aqueous solution which consists essentially of about 0.01% to about 0.1% CrO and having a free acid in the range of about 0.05 to about 1.0 and a total acid in the range of 0.5- 10.0, and as water is added thereto during use, maintaining the free acidity thereof within the above given range by adding thereto an acid selected from the group consisting of chromic acid and phosphoric acid, maintaining the CrO concentration within the above given range by adding thereto at least one hexavalent chromium ion-containing material and operating said rinse within the total acid concentration range of 0.5-10.0.

3. A method for maintaining an aqueous acidic hexavalent chromium containing solution in condition to enhance corrosion resistance of chemically coated metallic parts being continuously processed therethrough which comprises the steps of establishing an aqueous solution which consists essentially of about 0.01% to about 0.1% CrO and having a free acid in the range of about 0.05 to about 1.0 and a total acid in the range of 0.5 10.0, and as water is added thereto during use, maintaining the free acidity thereof within the above given range by adding thereto an acid selected from the group consisting of chromic acid and phosphoric acid, maintaining the CrO concentration within the above given range by adding thereto at least one hexavalent chromium ion-containing material selected from the group consisting of chromic acids and metallic dichromates and mixtures thereof, and operating said rinse within a total acid concentration range of 0.5-10.0.

4. A method for maintaining an aqueous acidic hexavalent chromium containing solution in condition to enhance corrosion resistance of metallic parts being con tinuously processed therethrough which comprises the steps of establishing an aqueous solution which consists essentially of about 0.04%-0.0'5% CrO and having a free acid in the range of 0.1-0.5 and, as water is added thereto, during use maintaining the free acidity thereof within the above given range by adding thereto an acid selected from the group consisting of chromic acid and phosphoric acid, maintaining the CIO;, concentration within the above given range by adding thereto a hexavalent, chromium ion-containing compound, and operating said rinse within a total acid concentration range of 37.

5. A method in accordance with claim 3 wherein said hexavalent chromium ion-containing compound is an admixture of CrO and calcium dichromate containing 10- 25% CrO and -90% calcium dichromate.

6. A method for maintaining an aqueous acidic hexavalent chromium containing rinse solution in condition to enhance the corrosion resistance of metallic parts being continually processed therethrough which oomprises the steps of establishing an aqueous solution consisting essentially of about 0.01% to about 0.1% CrO and having a free acid in the range of about 0.05 to about 1.0 and as water is added thereto during use maintaining the free acidity thereof within the above given range by adding a quantity of phosphoric acid sufiicient to neutralize the alkalinity of said water to a pH of 4.3, and maintaining the CrO concentration Within the above given range by adding thereto a hexavalent chromium ion-containing compound selected from the group consisting of G0,, and mixtures of Cr0 with at least one metallic dichromate, and operating said rinse within the total acid concentration range of 0.5-10.0.

7. A method in accordance with claim 6 wherein said alkalinity is neutralized by adding the equivalent of 75% phosphoric acid per liter of the said water added thereto with reference points as follows: 0.25 gram of 75% H PO for ppm. alkalinity water; 0.5 gram of 75% H PO for 200 p.p.m. alkalinity water; 0.75 gram of 75% H PO for 300 ppm. alkalinity water; and 1.0 grams of 75% H PO for 400 p.p.m. alkalinity water.

References Cited in the file of this patent UNITED STATES PATENTS 2,067,216 Thompson Ian. 12, 1937 2,233,422 Lodeeson Mar. 4, 1941 2,303,242 Tanner Nov. 24, 1942 2,318,656 Thompson May 11, 1943 2,391,656 Tanner Dec. 25, 4945 2,403,426 Douty et al. July 2, 1946 12,516,685 Douty et al. July 25, 1950 2,609,308 Gibson Sept. 2, 1952 2,882,189 Russell et al Apr. 14, 1959 

1. A METHOD FOR MAINTAINING AN AQUEOUS ACIDIC HEXAVALENT CHROMIUM CONTAINING SOLUTION IN CONDITION TO ENHANCE THE CORROSION RESISTANCE OF METALLIC PARTS BEING CONTINUOUSLY PROCESSED THERETHROUGH WHICH COMPRISES THE STEPS OF ESTABLISHING AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF ABOUT 0.01% TO ABOUT 0.1% CRO3 AND HAVING A FREE ACID IN THE RANGE OF ABOUT 0.05 TO ABOUT 1.0 AND A TOTAL ACID IN THE RANGE OF 0.5-10.0 AND AS WATER IS ADDED THERETO DURING USE, MAINTAINING THE FREE ACIDITY THEREOF WITHIN THE ABOVE GIVEN RANGE BY ADDING THERETO AN ACID SELECTED FROM THE GROUP CONSISTING OF CHROMIC ACIDS AND PHOSPHORIC ACIDS, MAINTAINING THE CRO3 CONCENTRATION WITHIN THE ABOVE GIVEN RANGE BY ADDING THERETO AT LEAST ONE HEXAVALENT CHROMIUM ION-CONTAINING MATERIAL AND OPERATING SAID RINSE WITHIN THE TOTAL ACID CONCENTRATION RANGE OF 0.5-10. 